Sara van den Berg

180 Chapter 7 change in CMV-specific antibody levels over a longer period of time within individuals, we used a median based linear model, the Theil-Sen estimator [41-43] with the R package ‘mblm’ (https://CRAN.R-project.org/package =mblm}). The slopes of the log-transformed antibody levels with age between all repeated measurements (at least 3 CMV+ time points) were calculated per individual for calculating the median of the slopes (representing the Theil-Sen estimator). This model is less affected by outliers or by skewed distributions than ordinary linear regression estimators. To estimate which variables are important predictors of CMV-specific antibody levels at endpoint, we performed a random forest prediction analysis with CMV-specific antibody levels at endpoint as dependent variable, using the random Forest R package [44]. The proportion of explained variance was calculated to estimate the prediction accuracy. The importance of the variables to predict CMV-specific antibody levels at endpoint, was shown a variable importance plot. Differences in T-cell subsets between the groups (CMV-, ST CMV+ and LT CMV+) were tested by one-way ANOVA and two groups comparisons were made using Tukey’s multiple comparison test. Frailty index differences were compared by non-parametric testing, Spearman correlation for comparing continuous variables, a Kruskal-Wallis test for comparison between groups, and post-hoc analysis for comparison between multiple groups by Tukey correction. Data were analyzed using SPSS statistics 22 for Windows (SPSS Inc., Chicago, IL, USA) and R 3.6.0 (https://www.rproject. org/). Serostatus Duration of CMV infection Total (n = 268) CMV - (n = 113) CMV + (n = 155) P -value * ST CMV + (n = 19) LT CMV + (n = 136) P -value * Age Age at baseline (years) 43.3 (6.6) 43.9 (7.1) 42.9 (6.2) 0.38 42.2 (7.5) 43 (6) 0.510 Age at endpoint (years) 70.9 (6.7) 71.7 (7.2) 70.6 (6.2) 0.32 70.4 (7.5) 70.6 (6.1) 0.780 Follow-up duration Total follow-up timespan (years) 27.7 (1.2) 27.7 (1.2) 27.7 (1.1) 0.61 28.2 (1.3) 27.6 (1.1) 0.030 Sex Women, % (n) 49.6 (133) 43.4 (49) 54.2 (84) 0.08 73.7 (14) 51.5 (70) 0.069 Education Low, % (n) 44 (118) 41.6 (47) 45.8 (71) 0.78 47.4 (9) 45.6 (62) 0.410 Middle, % (n) 27.6 (74) 28.3 (32) 27.1 (42) 36.8 (7) 25.7 (35) High, % (n) 28.4 (76) 30.1 (34) 27.1 (42) 15.8 (3) 28.7 (39) CMV serostatus CMV at baseline, % (n) 50.7 (136) CMV at endpoint, % (n) 57.8 (155) Note: Age and follow-up duration are summarized by mean (SD). *  Tested between serostatus groups Table 1: Summary statistics of the study population RESULTS Characteristics of the study population In total, 268 participants were included in this study (n=135 men, n=133 women), with an average age of 70.6 (60.3 – 88.6) years at end point ( Table 1 ). Follow-up time was 27.7 years on